1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device comprising a spacer with a novel structure for keeping a distance between a pair of substrates thereof between which liquid crystal compounds are scaled, uniformly.
2. Description of the Related Art
Liquid crystal display devices are coming into wide use as display devices for notebook type computers or monitors of computers which can display color images with high definition image qualities.
The liquid crystal display device of this sort has basically so-called liquid crystal display panel being constructed by at least one pair of substrates at least one of which is formed of a transparent glass or plastic substrate, or the like, and interposing liquid crystal compounds in a gap (simply called, liquid crystals, or a liquid crystal layer, also hereinafter) between the one pair of substrates which are arranged so as to confront one another with the gap. Moreover, these liquid crystal display devices of are classified roughly into either a simple matrix type liquid crystal display device which lights a predetermined pixel thereof up/out by applying a voltage selectively to various electrodes of the pixel provided in the liquid crystal display panel, or an active matrix type liquid crystal display device which lights a predetermined pixel thereof up/out by selecting an active clement for selecting the pixel, the active element being provided together with the various electrodes in the liquid crystal display panel.
The active matrix type liquid crystal display devices are represented by a type thereof using a thin film transistor (TFT) as the active matrix element constituting the liquid crystal display panel thereof. As the liquid crystal display devices comprising the liquid crystal display panels using the thin film transistors are thinly-made and lightweight, and provide high image qualities equal to those of cathode-ray tubes (Braun tubes), the liquid crystal display devices become widespread as monitors for displaying terminals of office automation apparatuses, or the like.
Displaying schemes for the liquid crystal display devices are classified broadly in accordance with driving methods of the liquid crystal display panels thereof, into following two sorts. One of them is an image-displaying scheme by interposing liquid crystal compounds between a pair of substrates each of which has a transparent electrode formed on, by driving liquid crystal compounds in accordance with voltages applied to the transparent electrodes, and by modulating incident light into the layer of liquid crystal compounds through the transparent electrodes, and this scheme is employed in most of a currently popular products.
Another of the displaying schemes displays an image by generating electric fields between a pair of electrodes provided on the same substrate of the liquid crystal display panel substantially in parallel to a surface of the substrate so as to drive the liquid crystal compounds between the electrodes thereby, and by modulating incident light into a layer of the liquid crystal compounds through a gap between the pair of the electrodes. The liquid crystal display panel of this displaying scheme is characterized by an image having remarkably wide viewing angle displayed thereby, and thus an image of an extremely high quality can be obtained by an active matrix type liquid crystal display device using this liquid crystal display panel. Characteristics of this displaying scheme are described in such documents as e.g. Japanese Patent Application Laid-Open No. 05-505247 (JP-A-505247/1993), Japanese Patent Publication No. 63-021907 (JP-A-021907/1988), Japanese Patent Application Laid-Open No. 06-160878 (JP-A-160878/1994). The liquid crystal display device of this displaying scheme will be called a liquid crystal display device of a lateral electric field scheme, hereinafter. The liquid crystal display device of the lateral electric field scheme is also called an IPS (In Plane Switching) type liquid crystal display device.
FIG. 17 is a cross sectional view for explaining an electric field generated in a liquid crystal display panel constituting the liquid crystal display device of the lateral electric field type. The liquid crystal display panel has a video signal line DL, a counter electrode CT, and a pixel electrode PX each of which is formed on one of substrates thereof SUB1, a protective film PSV being deposited on these layers, and an alignment control layer (also called, an alignment layer, or alignment film) ORI1 being formed at an interface with the liquid crystal layer LC. The liquid crystal display panel also has color filters FIL partitioned by a black matrix BM formed on another of the substrates thereof SUB2, an overcoat layer OC being deposited over these layers in order to prevent composing materials of the color filters and the black matrix from affect the liquid crystal compounds (called, liquid crystals, also hereinafter), and an alignment control layer ORI2 being formed-at an interface with the liquid crystal layer LC.
Insulating films GI, AOF are also formed on the one of the substrates SUB1. Moreover, the video signal line DL is formed of two conductive films d1 and d2 stacked on one another, the counter electrode is formed of a conductive film g1, and the pixel electrode PX is formed of a conductive film g2, wherein each of these is provided on the one of the substrates SUB1.
Furthermore, a distance between a pair of the substrates SUB1 and SUB2 (i.e. a thickness of the liquid crystal layer: a cell gap) is generally set at a predetermined value by dispersing inorganic-base or plastic-base spherical spacers (i.e. beads, but not shown in FIG. 17) between both of the substrates. On each outer surface of the substrates SUB1 and SUB2, a polarizer (a polarizer plate) POL1 or a polarizer (a polarizer plate) POL2 is stacked respectively.
On the other hand, Japanese Patent Application Laid-Open No. 09-073088 (JP-A-073088/1997) discloses conical spacers formed on the protective film on the color filter substrate (like a substrate SUB2, in FIG. 17) and columnar spacers formed by stacking color filter layers both of which are fixed to the color filter substrate, instead of such spherical spacers mentioned above regardless of the lateral electric field type liquid crystal display device.
The invention disclosed in Japanese Patent Application Laid-Open No. 09-073088 (JP-A-073088/1997) is to form conical spacers stably on the substrate so as to prevent deterioration of image contrast by light leaking through peripheries of the spherical spacers and displaying defects caused by spacers irregularly arranged during a step for spraying the spacers onto the substrate.
There are two problems to be solved by the present invention. First problem of the two problems is related to a design of pixels in the liquid crystal display panel of the lateral electric field scheme. For clarifying the problem, noise fields appearing in a area hidden by the black matrix will be mentioned.
In the liquid crystal display panel shown in FIG. 17, while an image is displayed by controlling orientation directions of liquid crystal molecules constituting the liquid crystal layer LC by applying an electric field EL generated substantially in parallel with the substrate between the pixel electrode PX and the counter electrode thereto, another electric field not contributing to displaying operation of the liquid crystal display panel (i.e. a noise field EN) is also generated between the video signal line DL and the counter electrode CT. If a space between these electrode is too narrow, an intensity of the noise field EN appearing in an area between the video signal line DL and the counter electrode CT becomes so large that the liquid crystals LC in the area are driven by the noise field and consequently permit undesirable light to penetrate in the area therethrough.
Unless the area between these electrodes CT, DL is shielded by the black matrix BM, the displaying quality of the liquid crystal display panel is deteriorated significantly. More specifically, deterioration of contrasts and cross talk phenomena appear in a screen of the liquid crystal display panel.
Means for reducing the influence of the noise field are conceivable as follows. One of them is to widen either a space between the video signal line DL and the counter electrode CT or a space between the video signal line DL and the pixel electrode PX. However, the pixel area cannot but be smaller by this means, and thus an aperture ratio thereof is so reduced that brightness thereof will be reduced.
Incidentally, the similar problem appears in another structure in which the video signal line DL is arranged adjacent to the pixel electrode PX, instead of the structure of FIG. 17 in which the video signal line DL is arranged adjacent to the counter electrode CT.
Second problem of the two problems appears in a case for increasing optical density of the black matrix BM in order to improve a light shielding (obstructing) property thereof against unnecessary transmitted light higher. However, a following problem will appear by increasing optical density of the black matrix BM.
First of all, metals like a Chromium (Cr) having a high light shielding property cannot be used for a black matrix BM of the liquid crystal display panel of the lateral electric field scheme, because the black matrix needs to have a high resistance in the liquid crystal display panel of the lateral electric field scheme (ref. c.g. Japanese Patent Application Laid-Open No. 09-043589 (JP-A-043589/1997)). Since, in the liquid crystal display panel of the lateral electric field scheme, the lateral electric fields being generated substantially in parallel with the substrate thereof for driving the liquid crystals are affected by the electrical property of the black matrix thereof, an electric field generated between the aforementioned electrode provided in the pixel and the black matrix keeps the lateral electric field from being generated ideally for driving the liquid crystals if the resistance of the black matrix BM is low. Consequently, such problems as declines of brightness and contrast of the liquid crystal display panel, reduction of a viewing angle thereof, etc. arise.
A pigment-dispersion type photosensitive resin is also utilized as a material for a black matrix BM other than metals. The pigment-dispersion type photosensitive resin has a sufficiently high resistance for avoiding the aforementioned problems, but can hardly increase a light-shielding property (a light-obstructing property, or optical density) thereof because pigments concentration in the photosensitive resin needs to be increased so higher in order to obtain higher optical density thereof that workability thereof in a photolithography process is deteriorated due to reduction of the resin concentration (the concentration ratio of the photosensitive resin against the pigments). More specifically, such problems as deterioration of exposing resolution, decrease of developing margin, a tendency to generate residues of pigments, etc. appear during the photolithography process for the black matrix using the photosensitive resin.
Furthermore, in another case for increasing the optical density of the black matrix by thickening a thickness thereof, flatness of the color filters is so deteriorated that a rubbing workability of the alignment control layer ORI2 is deteriorated, the cell gap can hardly maintain its uniformity, and consequently displaying quality defects of the liquid crystal display device like deterioration of a response time (a response rate) thereof appear.
The second problem for the present invention is related to a structure for forming spacers stably on a substrate of the liquid crystal display panel which constitutes the lateral electric field type liquid crystal display device. The spacers being formed stably on the substrate are called columnar spacers (a pillar spacers).
Since the spacers need to unify a thickness of the liquid crystal layer within a displaying screen of the liquid crystal display device, uniformity of respective film thickness of the columnar spacers is required. If the uniformity is not sufficient, brightness irregularity appears in a display screen of the liquid crystal display panel due to thickness deviation of the liquid crystal layer thereof. In a case for forming the columnar spacers on the substrate, it is difficult to make height of the columnar spacers even, because of a forming process thereof.
Generally, as the columnar spacers are formed by a series of steps for coating photosensitive resist on the color filter substrate or the TFT substrate, exposing the resist using a mask (masking exposure), and developing the exposed resist, dispersion of film thickness (height) of the columnar spacers appears in a substrate surface or in respective substrates due to non-uniformity of the coated photosensitive resist, non-uniformity of an optical irradiation intensity distribution in a substrate during the exposure step, non-uniformity of a development in the substrate during developing step in a substrate surface, etc. Under these circumstances, luminance irregularity due to non-uniformity of the cell gap appears in the display screen.
On the other hand, a mechanical characteristic of the spacers acquires importance for manufacturing the liquid crystal display panel. Since such composing layers as various electrodes, insulating films, thin film transistors, or the like are formed so as to overlap partially with each other on a substrate surface being opposite to the liquid crystal compounds in the liquid crystal display panel, an uppermost surface of the substrate is not even and has steps not higher than 1 xcexcm in height.
Therefore, the spacers have to be squeezed between the substrates or to be stuck into the composing layer formed on the substrate, when a pair of the substrates are aligned with one another so as to keep the cell gap therebetween uniform. Therefore, the columnar spacers are required to have mechanical properties as elasticity, hardness, etc. being equal to those of the spherical spacers.
However, the columnar spacers formed of such organic substances as the photosensitive resin can hardly obtain the mechanical properties equivalent to those of inorganic matter (e.g. silica, or else)-base spherical spacers or plastics-base spacers.
One of the objectives of the present invention is to solve each of the aforementioned problems, and to provide a liquid crystal display device using a liquid crystal display panel being free from deterioration of contrasts and brightness thereof, and the cross talking appearing therein, without decreasing an aperture ratio thereof, and despite the black matrix having relatively low optical density being used thereby.
For achieving the aforementioned objectives, one of the present inventions is characterized in that it defines resistivity or a dielectric constant (permittivity) of a columnar spacer being formed between a pair of substrates which constructing a liquid crystal display panel of a liquid crystal display device, in predetermined relationship to those of a liquid crystal compound in the liquid crystal display panel.
Moreover, another of the present inventions is characterized in that a structure of the columnar spacer includes a solid particle having a size substantially equal to a thickness of a liquid crystal layer (a cell gap) of the liquid crystal display panel.
Furthermore, the other of the present inventions is characterized in that a structure of the columnar spacer includes the solid particle having electrical conductivity, or another structure of the columnar spacer includes the solid particle together with a electrical conductive particle.
Representative structures of the present inventions are described as following structures (1) through (7).
Structure (1)
In a liquid crystal display device which comprises:
a liquid crystal display panel having a pair of substrates at least one of which is transparent, at least two kinds of color filters being formed on one of the pair of substrates, respective kinds of which have different colors from each other, a black matrix being provided between the color filters, a group of electrodes being formed on either one of the pair of substrates, a layer of liquid crystal compounds which have dielectric anisotropy and are sealed between the pair of substrates, and an alignment control layer for aligning molecules in the layer of the liquid crystal compounds; and
a driving means for applying driving voltage to the group of electrodes for displaying an image by the liquid crystal display panel,
a columnar spacer having smaller resistivity than that of the liquid crystal compounds is formed in a part of an area on the one of the pair of substrates, and the area is hidden by the black matrix.
According to the construction, a noise (electric) field is generated easily around the columnar spacer rather than in the liquid crystals, and thus a component of the noise field affecting a driving condition of the liquid crystals is decreased. Therefore, deterioration of contrast and brightness of the liquid crystal display panel and cross talks generated therein are suppressed even if a black matrix having relatively low optical density is used therein.
Structure (2)
In the structure (1), a relative dielectric constant (relative permittivity) of the columnar spacer is set to be larger than that of the liquid crystal compounds.
According to the construction, the noise field is generated easily around the columnar spacer rather than in the liquid crystals similarly to the structure (1), and thus a component of the noise field affecting a driving condition of the liquid crystals is decreased. Therefore, deterioration of contrast and brightness of the liquid crystal display panel and cross talk generated therein are suppressed even if a black matrix having relatively low optical density is used therein.
Structure (3)
In either one of the structures (1) and (2), solid particles of an inorganic matter base material (e.g., an inorganic material) or a plastic base material (e.g., an plastic, or a resin material) is mixed into the columnar spacer.
Generally, mechanical characteristics (elasticity, hardness, etc.) of the columnar spacers being formed of a photosensitive resist processed by exposure and development thereof are inferior to those of a spherical spacer formed of silica or hard plastic (called, a bead). Furthermore, non-uniformity of film thickness of the columnar spacers as finished is difficult to be avoided. According to the construction, uniformity of the cell gap can be kept by mixing solid particles similar to the aforementioned spherical beads into the columnar spacer and by adjusting the mechanical characteristics of the columnar spacer with a material and mixing amount of the solid particles. In addition, according to this construction, an optical leak due to the noise field can be reduced by adjusting electrical characteristics of the columnar spacer with a material of the spherical bead and amount thereof into the columnar spacer, also.
Structure (4)
In the structure (3), the solid particles are coated with an electrically conductive material (i.e. a conductive material). Silver particles is one of preferred conductive materials for this structure. An electrical characteristic of the columnar spacer is able to be controlled by mixing the solid particles coated with the conductive material into the columnar spacer, and light leakage is also avoided thereby. In this structure, the solid particles coated with the conductive material can be used together with spherical beads not coated with a conductive material, also.
Structure (5)
In either one of the structures (1) and (2), a mixture of solid particles of an inorganic matter base material (e.g., an inorganic material) or a plastic base material (e.g., an plastic, or a resin material) and particles of an electrically conductive material is mixed into the columnar spacer.
A silver particle is preferable for the conductive particle, and resistivity of the columnar spacer can be adjusted by mixing the conductive particles thereinto. Therefore, the noise field can be led into the columnar spacer side rather than the liquid crystal compounds side, and thus generation of the optical leakage can be suppressed.
Structure (6)
An average particle diameter (an average particle size) of the solid particles in the structure (3), the solid particles in the structure (4) on a surface of which is coated with a (electrically) conductive material, or the solid particles or the particles of the conductive material in the structure (5), is set to approximate a thickness of the layer of the liquid crystal compounds.
According to the construction, even if the columnar spacers cannot obtain uniform height thereof due to some thereof formed small in film thickness, the display defects like brightness irregularity in a displaying area of the liquid crystal panel can be suppressed by setting the cell gap to a desired value with any particle diameter of the aforementioned particles.
Structure (7)
In a liquid crystal display device which comprises:
a liquid crystal display panel having a pair of substrates at least one of which is transparent, at least two kinds of color filters being formed on one of the pair of substrates, respective kinds of which have different colors from each other, a black matrix being provided between the color filters, a group of electrodes being formed on either one of the pair of substrates, a layer of liquid crystal compounds which have dielectric anisotropy and are sealed between the pair of substrates, and an alignment control layer for aligning molecules in the layer of the liquid crystal compounds; and
a driving means for applying driving voltage to the group of electrodes for displaying an image by the liquid crystal display panel,
a columnar spacer having a smaller relative dielectric constant (smaller relative permittivity) than that of the liquid crystal compounds is provided so as to cover an area (preferably a whole area) between electrodes of the group of electrodes which is hidden by the black matrix.
According to the construction, the liquid crystal compounds are excluded from an area where the columnar spacer is formed, and thus the noise (electric) field is generated in the columnar spacer. Therefore, deterioration of contrast and brightness of the liquid crystal display panel and cross talks generated therein are suppressed even if a black matrix having relatively low optical density is used therein.
Structure (8)
In the structure (7), solid particles of an inorganic matter base material (e.g., an inorganic material) or a plastic base material (e.g., an plastic, or a resin material) is mixed into the columnar spacer.
Structure (9)
In the structure (8), the solid particles are coated with an electrically conductive material (i.e. a conductive material). Silver particles is one of preferred conductive materials for this structure. In this structure, the solid particles coated with the conductive material can be used together with spherical beads not coated with a conductive material, also.
Structure (10)
In the structures (8), a mixture of solid particles of an inorganic matter base material (e.g., an inorganic material) or a plastic base material (e.g., an plastic, or a resin material) and particles of an electrically conductive material is mixed into the columnar spacer.
Structure (11)
An average particle diameter (an average particle size) of the solid particles in the structure (8), the solid particles in the structure (9) on a surface of which is coated with an (electrically) conductive material, or the solid particles or the particles of the conductive material in the structure (10), is set to approximate a thickness of the layer of the liquid crystal compounds.
Advantages due to the constructions (8)-(10) are similar to those described in accordance with the aforementioned constructions (3)-(5), respectively.
Incidentally, the present invention is not limited to any of the above-described structures and the structures of embodiments which will be described below, and it goes without saying that various modifications can be made without departing from the technical ideas of the present invention.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.